Conventionally, a known thermal-type airflow measuring device is employed for measuring a flow rate of air by utilizing heat transfer with air. Such a conventional airflow measuring device is, for example, equipped to an air intake passage of an internal combustion engine for measuring a flow rate (intake air amount) of intake air drawn into the internal combustion engine.
Specifically, for example, an airflow measuring device receives a part of air-intake mainstream passing through an air intake passage and sends an electric signal according to the intake air amount. The airflow measuring device includes a case and a sensor chip. The case forms a bypass passage therein to flow air, which enters from the air intake passage, therethrough. The sensor chip generates an electric signal according to heat transfer caused with air flowing through the bypass passage. In the airflow measuring device, the sensor chip is not located directly in the air intake passage, through which the air-intake mainstream passes, but is located in the bypass passage. With this configuration, the airflow measuring device is enabled to obtain a detection result with a small fluctuation, without being exerted directly with an influence of turbulence of the air-intake mainstream passing through the air intake passage.
The air-intake mainstream causes pulsation inevitably due to opening and closing operation of a valve equipped to the internal combustion engine. Therefore, the intake air amount fluctuates between the peak values of the pulsation. As described above, the airflow measuring device employs the thermal-type measuring configuration to obtain detection value by utilizing heat transfer with air. Inherently, because of the thermal-type measuring configuration, the detection value of the airflow measuring device becomes lower than the center of the pulsation representing the true value of the intake air amount. Consequently, the detection value of the airflow measuring device has a detection error in negative value. In consideration of this, the airflow measuring device has a configuration with a predetermined relationship between a passage length L2 of the bypass passage and a passage length L1 of the air intake passage, through which air flows straight without passing through the bypass passage. Specifically, the passage length L2 is set to be greater than the passage length L1 thereby to set a bias width of the detection value corresponding to a value L2/L1. In this way, the detection value is biased, i.e., increased by the bias width to reduce the detection error in negative value.
It is noted that, the detection error in negative value varies correspondingly to the intake air amount. Specifically, the detection error in negative value increases in degree, i.e., the detection error in negative value decreases in value, as the intake air amount increases. It is conceivable to set the value L2/L1 so that the detection error in negative value becomes zero when the intake air amount is a specific large value. Nevertheless, in this case, when the intake air amount decreases from the specific large value, the bias width corresponding to the value L2/L1 may be excessively large, while the detection error in negative value decreases. Consequently, the decreased detection error in negative value is cancelled with the excessive bias width to cause a detection error in positive value in the detection value.
Therefore, when a flow rate of a pulsating flow is measured with a thermal-type airflow measuring device, an error cancelable range exists in the detection range of flow rate, corresponding to the value L2/L1. Within the error cancelable range, detection error in negative value and detection error in positive value can be cancelled, without excess and deficiency. That is, when the flow rate decreases below the error cancelable range, detection error in positive value may occur in the detection value.
An airflow measuring device of DE 10 2008 042 807 A1 (Patent Document 1) has a configuration to restrain a swirl flow from occurring along the outer wall surface of a case at the downstream of an outlet of a bypass passage. Specifically, in Patent Document 1, the outer wall surface of the case of the airflow measuring device is equipped with two ribs. The two ribs are in parallel with the mainstream of intake air and surround the outlet of the bypass passage. A lid is further provided at the downstream of the outlet to bridge the two ribs. According to Patent Document 1, a swirl flow can be restrained to rectify flow along the outer wall surface of the case at the downstream of the outlet. The configuration of Patent Document 1 may restrain occurrence of a swirl flow, nevertheless, Patent Document 1 cannot address the detection error in positive value caused by decrease in intake air amount.